This invention relates generally to a more precise, reliable, and continuous navigation system using the GPS or GLONASS satellite based radionavigation system and more particularly to use of this navigation system as a precision approach and landing system for aircraft.
The Global Positioning System (GPS) is a widely used satellite-based navigation system consisting of a network of satellites broadcasting pseudo-random noise (PRN) codes modulated on an L-band carrier (1575.43 MHz). A GPS receiver uses measurements of the PRN code-phase from four or more satellites to solve for the three-dimensional position of the receiver and to calibrate its internal time reference. The GPS receiver determines velocity from measurements of the carrier phase and doppler. Accuracy of the GPS solution is limited by the errors on the GPS signals and the geometry established by the positions of the satellites relative to the user. Presently, neither the precision nor the coverage of the standard positioning service provided by the proposed 21-satellite constellation meets the requirements for a precision approach and landing system for aircraft.
A common method for improving the precision of the standard positioning service is to broadcast differential corrections to users of the service. A standard message format for these corrections is described in "RCTM Recommended Standards for Differential Navstar GPS Service," Version 2.0, RTCM Special Commitee No. 104, January, 1990. This method is used in the landing assistance system described in U.S. Pat. No. 4,894,655 to Joquet et al. In accordance with these prior art teachings, the GPS accuracy is improved through the transmission of differential GPS corrections on a radio channel according to the standards of the microwave landing system known as MLS. However, this method requires that the user aircraft carry both an MLS and a GPS receiver. Moreover, this method does not address the problem that the GPS satellite coverage is insufficient to provide a continuous and reliable precision approach and landing service. See Braff, R. and R. Loh, "Analysis of Stand-Alone Differential GPS for Precision Approach," Proceeding of the RION Satellite Navigation Conference, London, England, November, 1991. Differential GPS flight test results have also demonstrated that the accuracy provided using this method is only sufficient to meet a relaxed "Near CAT I" precision approach requirement. See Braff et al., supra, and L. Hogle, "Investigation of the Potential Application of GPS for Precision Approaches," NAVIGATION, The Journal of the Institute of Navigation, Vol. 35, No. 2, Fall, 1988.
Test results documented in the prior art have demonstrated that GPS can provide sufficient accuracy to meet precision approach and landing system requirements using the GPS carrier phase data to solve for the aircraft's position. See Landau, H. and G. Hein, "Precise Real-Time Differential GPS Positioning Using On-the-fly Ambiguity Resolution," Proceedings of the RION Satellite Navigation Conference, London, England, November, 1991. This processing method is generally termed "Kinematic GPS" or "Carrier-Ranging" in the literature. In the prior art, GPS carrier measurements from a ground-based reference receiver and the airborne receiver are processed to solve for the precise relative position of the aircraft with respect to the ground facility. Test results have demonstrated real-time positioning accuracies of better than 10 cm using the method described by Landau et al., which is sufficient to meet CAT I, II, and III precision approach accuracy requirements. However, the GPS satellite constellation does not provide sufficient coverage and redundancy to meet these operational requirements for a precision approach and landing system.
One solution that has been proposed by the RTCM Special Committee No. 104 and others to improve the GPS satellite coverage is to augment the GPS satellite measurements with a range observation from a ground-based transmitter, i.e. a pseudolite. In accordance with the teachings of the prior art, pseudolites have been proposed that broadcast a signal at the same frequency as GPS (1575.42 MHz) so that the aircraft receiver can process this measurement as though it were another satellite. However, a pseudolite with this signal format will also act as a jammer to users operating near the transmitter, thereby preventing the receiver from tracking the GPS satellites. This interference problem renders this technique unacceptable for use in a precision approach and landing system. The SC-104 reference, supra, and A.J. Van Dierendonck, "Concepts for Replacing Shipboard Tacan with Differential GPS," ION Satellite Division Third International Technical Meeting, September, 1990, describe a time-slotted signal structure for a pseudolite which somewhat alleviates the foregoing problem. However, this pseudolite signal will still jam satellite signals at close range. Moreover, the time-slotted or pulsed signal format does not allow contiguous carrier phase measurements to be made of the pseudolite signal. This means that the pseudolite signal cannot be included in the carrier-ranging navigation solution, and the time-slotting also affects the use of the pseudolite signal as a high-rate communication link for differential corrections.
To avoid the possibility of the pseudolite signal jamming the satellite signals, the pseudolite signal can be broadcast at a different frequency from that of the GPS satellites. This is a similar approach to that described in U.S. Pat. No. 4,866,450 to Chisholm wherein a ranging reference signal modulated with correction data is broadcast from a ground-based transmitter synchronized with GPS time. However, in accordance with the teachings of Chisholm, the signal is again time-slotted and so has the same disadvantages as the pseudolite design described in the SC-104 reference, supra. Another disadvantage of the method described in the Chisholm patent is that a second receiver is required in the aircraft to process the ground station signals broadcast at the second frequency. The timing and frequency offsets between the GPS and second receiver will introduce a significant offset between the range measurements made by the two receivers. Although the additional measurement will improve the solution geometry, the receiver offset will degrade the performance of the differential solution. Therefore, it is not believed that the teachings of Chisholm represent an improvement over those of Joquet et al. Based on test results, both of these methods will only meet a relaxed "Near CAT I" precision approach requirement, as described in Braff et al., supra.